Abstract Triple-negative breast cancer (TNBC) comprises 15 to 20% of breast cancers and is the most aggressive subtype with a significantly shorter median overall survival compared to other subtypes. There are no targeted therapies for TNBC and only 10% of TNBC patients respond to immune therapy. Thus, most of patients with TNBC still mainly depend on conventional chemotherapies, with doxorubicin (Dox) as a commonly used one. The two crucial concerns with use of chemotherapies such as Dox are severe toxicity and drug resistance. Extensive studies have unveiled many altered signaling pathways that contribute to the development of Dox resistance. However, it is a daunting task to identify agents that can target the diverse drug-resistant pathways simultaneously. Recently, synthetic miRNA mimics or inhibitors have become attractive tools to battle cancer as a new type of therapies or to break drug resistance because one microRNA can target multiple genes in multiple signaling pathways. Our recent published data showed that miR-489 is lost in a majority of breast cancers especially TNBC. Loss of miR-489 confers resistance to chemotherapies such as doxorubicin (Dox) and restoration of miR-489 reverses Dox resistance both in vitro and in vivo. Further mechanistic studies revealed that miR489 can inhibit Dox-induced cytoprotective autophagy, increase Dox localization in nucleus and potentiate Dox-induced ATP release. Importantly, we developed a nanoparticle system to specifically deliver miR489 to breast tumors. Based on these findings, we propose that nanoparticle delivered miR-489 can simultaneously modulate multiple pathways involved in cell proliferation, apoptosis, epithelial- mesenchymal transition (EMT), autophagy and ER stress to enhance Dox-induced cell death and anti- cancer immunity and therefore delay or reverse Dox resistance. Three specific aims are proposed. SA1. To formulate tumor-targeting miR-489 nanoparticles and characterize the delivery efficiency and toxicity both in vitro and in vivo. SA2. To test whether miR-489 synergizes with Dox to induce cell death and reverses Dox-resistance using cell line and patient-derived xenograft (PDX) mouse models. SA3. To investigate whether miR-489 enhances the efficacy of Dox treatment of PDX using humanized mouse models. Overall, this study aims to develop miR-489 as a novel therapeutic agent to reverse Dox resistance and thus enhance the efficacy of Dox-based chemotherapy.